1. Field of the Invention
The present invention relates to an objective lens driving device for an optical pickup, and more particularly to an objective lens driving device in which a plurality of suspensions support an objective lens bobbin.
2. Description of the Prior Art
A light beam emitted from a built-in light source such as a semiconductor laser element is converged by an objective lens and other optical devices such as a collimator lens or a beam splitter on a signal recording surface of an optical disk as the optical recording medium. The light beam reflected by the signal previously recording surface is detected by a photo detector to produce the data signal recorded on the optical disk, or in which desired data signals are recorded by converging a light beam on the signal recording surface. With this type of optical pickup device, the objective lens for converging the light beam on the signal recording surface of the optical disk is movably supported in two directions orthogonal to each other, that is, in a direction along an optical axis (or in a focusing direction) and in a direction orthogonal to the optical axis (or in a tracking direction).
As an example of such the objective lens driving device, FIG. 1 is a perspective view of a conventional objective lens driving device which is similar to that disclosed in Japanese Utility Model Application No. 62-000787. Referring to FIG. 1, reference numeral 1 designates an objective lens opposed to an optical disc (not shown). The objective lens 101 is held by objective lens bobbin 102. The objective lens bobbin 102 is movably supported by four supporting rods 103 (hereinafter called suspensions) in the focusing direction and the tracking direction.
Each suspension 103 is made of elastic material such as rubber or elastomer. A focusing coil 106 is wound around a rectangular outer periphery of the objective lens bobbin 102, and tracking coil 107 is fixed to an outer periphery of the focusing coil 106. A pair of magnets 105 retained by yokes 108 are located outside the tracking coil 107 in such a manner as to be opposed to the focusing coil 106 and the tracking coil 107.
In the conventional objective lens driving device as mentioned above, the objective lens bobbin 102 is driven by the current flowing in the focusing coil 106 and magnetism of the magnets 105 to by that deform the suspensions 103 and affect the focusing control in a direction (f-direction) of an optical axis of the objective lens 101. In such a focusing control operation, a detected beam to be irradiated from the objective lens 101 onto an information recording surface of the optical disc is controlled to be focused on the recording surface.
Further, the objective lens bobbin 102 is driven by the current flowing in both side portions of tracking coil 107 and the magnetism of the magnets 105 to deform the suspensions 103 and affect the tracking control in a direction (t-direction) along the information recorded surface of the optical disc. In such a tracking controlling operation, a spot of the detected beam to be irradiated from the objective lens 101 onto the information recorded surface of the optical disc is controlled to precisely scan an information track of the recorded surface.
However, since the objective lens bobbin 102 is supported by the suspensions 103 made of elastic material such as rubber or elastomer, the suspensions 103 are subject to vibration occurred in driving the objective lens bobbin 102, and then the vibration of the suspensions 103 is transmitted to the objective lens bobbin 102.
The objective lens driving device which improves the above problem is disclosed in U.S. Pat. No. 5144607.
FIG. 2A is a perspective view of the other conventional objective lens driving device, and FIG. 2B is an exploded perspective view showing the objective lens driving device shown in FIG. 2A.
Referring to FIGS. 2A and 2B, the objective lens driving device includes an objective lens 201 supported by the objective lens bobbin 202, an objective lens supporting member 203 supporting the objective lens bobbin 202 for swinging movement, focusing coil 206 and tracking coil 207 mounted to the objective lens bobbin 202, magnets 205 mounted for facing the coil 206 and 207 and a magnetic yoke 204 supporting the magnets 205.
A coil housing section 209 is provided on both sides of the objective lens bobbin 202. The coil housing section 209 of the objective lens bobbin 202 is provided on both sides thereof with apertures 210 and the tracking coil 207 may be introduced into the coil housing section 209.
Thus, both tracking coils 207 are mounted to front and rear sides of the objective lens bobbin 201. The focusing coils 206 are mounted on the outer lateral sides of these tracking coil 207.
The objective lens supporting members 203, supporting the objective lens 201 for swinging movement in two directions at right angles with each other, is made from a thin metal sheet, such as a stainless steel sheet.
The objective lens bobbin 202 is supported by the objective lens supporting members 203 within the magnetic yoke 204 arranged as a rectangular frame so that the lens bobbin 202 faces the magnets 205 mounted within the magnetic yoke 204. The side walls of the magnetic yoke 204 are connected to one another such as by welding to complete a rectangular frame.
When the focusing driving current is supplied to the focusing coil 206, the objective lens bobbin 202 is driven so as to be deflected along the optical axis of the objective lens 201 shown by an arrow f in FIG. 2A, that is, in the focusing direction.
On the other hand, when the tracking driving current is supplied to the tracking coil 207, the objective lens bobbin 202 is driven so as to be deflected along the direction normal to the optical axis of the objective lens 201 shown by an arrow t in FIG. 2A, that is, in the tracking direction. However, as shown in FIG. 2A, construction of the above objective lens driving device is very complex. Therefore, the production cost of the objective lens driving device is high.
In addition, since the objective lens bobbin 202 is supported by the objective lens supporting member 203 made from stainless steel sheet, the objective lens supporting member 203 is subject to vibrations occurred in driving the objective lens bobbin 202, accordingly, the vibrations of the objective lens supporting members 203 can be transmitted to objective lens bobbin 202.
Accordingly, although the objective lens driving device showed in FIG. 2A has been improved over the prior art objective lens driving device shown in FIG. 1 (Japanese Utility Model Application No. 62-00787), the above driving devices cannot accurately do the focusing and the tracking controlling operations of the objective lens.
In addition, in order to drive the objective lens bobbins 102 and 202 along the focusing and the tracking directions in the objective lens bobbins, in the former, the suspensions 103 made of elastic member such as rubber or elastomer is deflected. In the latter, the objective lens supporting member 203 made from a stainless steel sheet is deflected. As a result, the objective lens driving devices will require a lot of driving current.